The automotive and light truck industries depend heavily upon catalytic converters to control exhaust emissions. These catalytic converters typically utilize ceramic substrates in the form of honeycombs coated with platinum and/or palladium to catalyze hydrocarbons, carbon monoxide, and nitrogen oxides in the exhaust into carbon dioxide, nitrogen, oxygen, and water.
During manufacture, it is important that the catalytic converters be gas tight. That is, that the converters, when coupled into an exhaust system, do not leak. If a converter leaks, then the noxious gasses the converter is intended to catalyze may pass directly into the atmosphere, thereby defeating the purpose of the converter.
A leaky converter may also pass air into the converter. Exhaust systems run hot. Therefore, the passage of much cooler air into the converter may cause fracturing and other damage to the ceramic substrate. This damage may reduce the converter efficiency, and may ultimately lead to converter failure.
Various techniques are used to assure that a converter is gas tight. Among these techniques are rolled seams and metal inert gas (MIG) welding. Both rolled seams and MIG welding pose problems during manufacture.
Rolled seams are typically used where workpiece edges are readily available. One such place is in the manufacture of a shell to contain the ceramic substrate. When the shell is formed into a cylindroid shape, the edges may be rolled together and flattened to form a gas-tight seam. Once formed, the seam is often spot-welded to inhibit unrolling due to thermal expansion and contraction, thereby maintaining the necessary gas-tight status.
A disadvantage of a rolled seam is that the seam is necessarily thick. This produces a discontinuity in the shell that inhibits the proper joining of other components.
Where edges are not readily accessible, as during final assembly, MIG welding is often used. MIG welding is a modified form of arc welding. In MIG welding, direct current is passed though a welding torch having a continuous metal wire as the welding element. An arc is generated, which melts the wire and the metal workpiece. This produces a molten pool, which creates the weld.
Gas is fed to the torch during the welding process. This gas acts as a fluxing agent. The use of gas as a fluxing agent inhibits oxidization of the weld and surrounding metal. The use of gas also eliminates the production of a flux residue.
MIG welding is often used in the welding of thin sheet metals, such as mild steel, stainless steel, and aluminum. Thus, MIG welding is common in the automotive and light truck industries, where extensive use is made of various sheet metals.
MIG welding is a filling process, i.e., the arc cuts a groove which is filled with molten metal from the wire and the surrounding workpiece. MIG welding is therefore suitable for the butt-welding of thin materials. Such butt welds are prone to minor defects of the weld (the bead). From the point of view of strength, these minor defects tend to be negligible. From the point of view of gas tightness, however, these minor defects may produce leaks over time. This is especially so where the welded object is subject to repetitive radical thermal differentials. This is exactly the case with catalytic converters. MIG welding, though common, may develop leaks and severely limit the overall life of a catalytic converter.
Laser welding has been attempted with mixed success. While an effective welding technique, laser welding has resulted in poor quality control because of excessive penetration (i.e., piercing). Piercing often results in seals that are not gas tight, or in weak gas-tight seals leading to early failure during operation.